Ion propulsion device



R. J. SUNDERLAND ETAL 3,263,415

Aug. 2, 1966 ION PROPULSION DEVICE 2 Sheets-Sheet 1 Filed March 6. 1961 INVENTOR. JOHN R. RADBILL ROBERT J SUNDERLAND ATTORNE 1966 V R. J. SUNDERLAND ETAL 3,263,415

' ION PROPULSION DEVICE Filed March 6. 1961 2 Sheets-Sheet 2 INVENTOR. 4 JOHN R. RADBILL ROBERT J. SUNDERLAND AT TORN EY E1 1111mm N Ur HHUHH "iii United States Patent 3,263,415 ION PROPULSION DEVICE Robert J. Sunderland, Gleudora, and John R. Radbill,

Azusa, Califl, assignors to Aerojet-General Corporatron, Azusa, Califl, a corporation of Ohio Filed Mar. 6, 1961, Ser. N .'93,794 13 Claims. (Cl. 6035.5)

This invention relates generally to a propulsion device and more particularly to an ion propulsion engine. Ion propulsion engines appear to be particularly useful in a gravity free environment where a low thrust level continued for long periods of time can produce an extremely high velocity. These engines operate by introducing a source of ions into an electric field which accelerates them to the desired ejection velocity. The thrust of such an engine depends on the number and the velocity of the ejected ions.

Ion engines of the surface contact type operate by directing a material with a low ionization potential such as cesium or rubidium vapor, into contact with a heated metal surface with a high work function, such as tungsten. This contact causes the initially neutral vapor atoms to become ionized so they respond to a suitable electric field and are accelerated out of the motor to provide thrust. In prior surface contact engines, an extracting grid or electrode at a suitable potential was placed near the metal surface to provide the desired electric field in a diodetype arrangement. The metal surface was either planar or it was provided 'with a slight curvature to better focus the ion beam. In addition, the ion emission was from a single surface.

The electric field between the extracting grid and the tungsten surface pulled the ions away from the metal sur face. The resulting ion current was limited by Childs Law. This means that the current density was restricted to the three halves power of the extracting voltage. In addition, because of restrictions imposed by the space charge, the current in the conventional surface contact device at grid potentials below voltage saturation was independent of emission temperature. As a result, in order to get an increase in the current how, it was necessary to increase the potential on the extracting electrode. This was objectionable because it increased the danger of electrical breakdown. Consequently the ion current attainable in practice was very small and the obtainable thrust from each modular unit was so low that a very large number of modular units would be necessary to achieve a useful thrust level.

Another problem connected with the prior diode type arrangement was the difficulty in focusing the ion beam leaving the emitting surfaces. To overcome this difficulty, complex electromagnetic focusing devices were required. This imposed a further objectionable weight penalty.

It is apparent that it would be desirable to provide a surface contact ion engine which develops a current density greater than the limitations predicted by Childs Law and which is designed so the influence of the space charge on the ion current is unimportant within the normal operating voltage range of the motor. It would also be desirable if the motor were designed so the ion current can increase with temperature in the normal operating voltage range in order to reduce voltage requirements.

Additionally, it would be desirable to provide a surface contact ion motor wherein the configuration of the emitter inherently produces a narrow ion beam without requiring other electromagnetic focusing equipment.

What is needed, therefore, and comprises an important object of this invention is an improved surface contact ion motor having the above described features.

Patented August 2, 1966 In its principal aspect, the improved ion propulsion engine comprises a housing having an enclosed cavity formed therein. The cavity is provided with a vapor inlet and a small exit aperture. For the production of positive ions, the walls of the cavity are fabricated from a material having a high work function and the cavity is maintained at a high temperature. A vapor having a low ionization potential is introduced in the housing cavity causing the atoms of the vapor to become ionized by contact with the cavity walls. An extracting electrode at a suitable potential is placed in front of the aperture to provide an electric field. Wit-h this arrangement, a portion of the electric field penetrates the exit aperture whereby the ions in the cavity are guided out of the cavity by the electric field. The ions leaving the cavity are accelerated by the extracting grid. A grounded decelerating electrode positioned beyond the extracting electrode determines the desired ion velocity and hence determine-s the specific impulse. The momentum of the ion beam provides thrust. For the production of negative ions, the walls of the housing cavity would have to be made from a material having a work function which is lower than the electron affinity of thematerial to be ionized.

Other objects of this invention will become more apparent when read in the light of the accompanying specifications and drawings wherein:

FIG. 1 is a side sectional view of a cavity ion modular' unit constructed according to the principles of this invention;

FIG. 2 shows the equipotenti-al lines produced by the electric field formed inside the cavity, due solely to the space charge;

FIG. 3 is a view of the equipotent-ial lines inside the cavity caused by an external electric field in the absence of a space charge;

FIG. 4 is a sectional view showing the equipotential lines caused by the electric field inside the cavity due to internal space charge and by the external field and showing in dotted lines the path followed by the ions in the housing as they leave the cavity;

FIG. 5 is a perspective view of an ion propulsion engine formed from a number-of cavity ion sources or modular units combined together to increase the available thrust;

FIG. 6 is a side sectional view of the ion propulsion engine shown in 'FIG. 5;

FIG. 7 is a control circuit for controlling the temperature of the walls of the cavity and the temperature of the ovens; and

FIG. 8 is a schematic diagram showing a typical voltage relationship between the housings producing positive and negative ions and the voltages on their extracting electrodes.

Referring now to FIG. 1 of the drawings, an ion propulsion device indicated generally at 10 comprises an elongated thin walled tube 12 having ends 14 and 16. A housing 18 is mounted on tube end 16 by any suitable means. This housing has a space or cavity 20 for-med therein and is provided with an inlet portion 22 and a small exit portion or aperture 24. The inlet portion 22 of housing '18 communicates with the end 16 of the tube 12. :For the production of positive ions, the walls of the housing cavity are formed from a material "having a high work function and a highrnelting point such as tungsten.

An oven such as oven 26 is connected totube end 14 (see FIG. 6). The oven is provided with a heating means which, in this embodiment, comprises high resistance wires 28 mounted therein by any suitable means. The high resistance wires 28 are connected to an electrical power source 30 (see FIG. 7 Thetemperature of the oven may be controlled by using a conventional saturable core reactor 32 to control the current fiow-i-n the high resistance wire-s 28. In this way, a small variation in the magnitude of a D.-C. control voltage, indicated generally by the reference numeral 34, results in a large variation in the current fiow through the high resistance wires 28 in a manner well known in the art. With this arrangement, the temperature of oven 26 (see FIG. 6), may be raised high enough to vaporize the material therein. In addition, variations in oven temperature can be used to control the flow of vapor to housing 18. Cesium, because of its low ionization potential, is inserted in the oven 26, for reasons to become apparent below. It is understood, however, that other materials having low ionization potentials are contemplated and may be used. If cesium is in the oven, the temperature of the oven is set in the neighborhood of 130 C. This temperature vaporizes the cesium and has been found suitable for ion production. When the cesium is vaporized, it flows from the oven 26 through tube 12 to the inlet portion 22 of housing 18. A tungsten battle 36 (see FIG. 1) is mounted inside the housing cavity adjacent the inlet portion 22 for the purpose of deflecting the cesium vapor and distributing it over the tungsten surfaces of the cavity walls.

In order for ionization to take place, the walls of housing cavity must be heated. A temperature in the neighborhood of 1450 K. has been found satisfactory. This temperature level can be reached a number of ways. For example, the housing walls can be heated by a fluid which is itself heated by a nuclear reactor. In this embodiment, however, the heating is done electrically. As seen in FIG. 1, the temperature of the cavity walls is raised by means of a high resistance tungsten wire 38 mounted inside the cavity by any suitable means. One end of wire 38 extends through the walls of the housing and is connected to a terminal 40. The other end of the wire is connected to one end of power line 42, inside the cavity. Power line 42 is mounted inside a metal probe tube 44 and is insulated therefrom. The probe tube 44 is mounted inside tube 12 by spiders 45 for reasons to be described below.

As seen in FIG. 1, the thin wall tube 12 in this particular embodiment is formed in two parts connected together by a coupling nut 46. The opposite end of power line 42 leaves tube 44 and tube 12 through a gas tight passage in the thicker walls of the coupling nut for connection to a power source (not shown). In this way, by passing a suitable electric current through the tungsten wire 38 the temperature of the walls of the cavity in housing 18 may be raised. An electrical power source and control circuit such as that shown in FIG. 7 may be connected to the tungsten wire 38 for varying the temperature of the walls of the cavity, for reasons to be described below. With this arrangement, contact between the neutral cesium atoms and the tungsten walls of the cavity or the surface of the tungsten heating wire resultsin the production of positively charged cesium ions.

A ceramic sleeve 48 may be fitted over the outer walls of housing 18 serving both to retain heat in the housing and to provide a support for extracting electrodes or grids 50 and 51. Electrode or grid 50 is placed in front of the exit aperture 24 and in this embodiment a potential of +2 kv. is applied to the housing 18 by contact 21. (See FIG. 8.) A suitable electric potential in the neighborhood of -2 kv. may be applied to the electrode 50 to provide a 4 kv. extracting voltage to the housing cavity 20. This potential applied to grid 50 creates an electric field between the electrode 50 and the exit aperture 24 of housing cavity 20.

As seen in FIG. 2, the formation of ions inside the housing cavity 20, in the absence of an external electric field, creates a space charge electric field inside the cavity as shown by the equipotential lines 52 in FIG. 2. This space charge field creates a potential hill in the center of the cavity which repels the ions and causes them to remain near the walls of the cavity.

As shown in FIG. 3, the external electric field caused by the potential on electrode 50, and in the absence of a space charge electric field, penetrates the exit aperture 24, forming thereby generally semi-circular equipotential lines 53 inside the cavity around the aperture 24. When both a space charge field and a portion of an externally applied electric field are present inside the cavity, the equipotential lines from each field are distorted somewhat as shown in FIG. 4.

If the externally applied electric field has the correct sign or polarity with respect to the charge on the ions in the cavity, it will cooperate with the space charge field and seem like a potential valley to the ions in the cavity. Since the ions move generally perpendicular to the equipotential lines and move from a high potential to a low potential, ions present in the cavity will move down the potential hill due to the space charge electric field, and into the potential valley created by the portion of the externally applied electric field penetrating the housing cavity. These electric fields inherently guide the ions through exit aperture 24 and out of the housing where they will be further accelerated by electrode 50.

Since the exit aperture 24 is small, the ions leave the cavity in a narrow high density beam. Consequently the combination of the electric fields and the small exit aperture produces a guiding and focusing effect on the ions. This eliminates the need for auxiliary and expensive ion focusing devices to perform the same function. In addition, the cavity ion source will produce an ion current in excess of the limitation imposed by Childs Law.

A grounded electrode 51 is mounted on the ceramic sleeve 48 beyond the electrode 50. The potential dif ference between electrode 51 and the potential applied to the housing determines the specific impulse of the ions leaving the propulsion device Electrode 51 is grounded to prevent electrically charged particles beyond the electrode from being influenced by the potentials on the electrode 50 in housing 18.

It is further noted that although the ions are guided out of the cavity through the small exit aperture 24 by the electric fields, the neutral atoms can escape through this exit aperture only if they happen to be moving in the proper direction. Since the exit aperture is comparatively small, the number of neutral atoms escaping this way will be insignificant. As a result, substantially all the neutral atoms are retained in the cavity in position to become ionized. In this way, the ionization efiiciency of the ion propulsion device is substantially increased.

In addition, the configuration of the space charge field inside the cavity may be adjusted by proper attention to the geometry of the cavity or by the insertion of a potential probe inside the cavity. Such an adjustment of the space charge field may be desirable in order to increase the cooperation between the space charge field and the portion of the externally applied field penetrating the cavity for the purpose of expediting the movement of the ions out of the cavity. In the present embodiment the probe comprises a metal tube 44. One end of the probe is adapted to be connected to a source of suitable potential at contact 55. The other end of the probe terminates in the housing cavity at a point selected to optimize the space charge field for best cooperation with the externally applied electric field from electrode 50 (see FIG. 1).

The production of ions in housing cavity 20 is temperature sensitive so that by changing the temperature of the Walls of the cavity and at the same time adjusting the flow of neutral particles into the cavity by varying the temperature of the oven, the number of ions and the density of the ion beam produced may be changed. As a result the thrust of the propulsion device can be varied without changing the voltage on electrode 50 or, as will become apparent below, without disturbing the ion beam distribution. A large number of these propulsion devices or units must be combined to provide a use ber of units involved would have to be much more complicated. This would increase the likelihood of an electrical breakdown.

In the present embodiment, electrical means are used to heat the housing. Alternatively, as stated above, fluids heated by nuclear fission may be used to heat the housing walls directly. With such an arrangement, the temperature of the cavity walls in the housing could be controlled by controlling the rate of nuclear fission. This would further reduce the dependence of this propulsion system on an electrical power source.

In order to produce negative ions, a housing 23, formed from platinum or tantalum is provided with a housing cavity 25 (see FIG. 8). The walls of the housing cavity may be coated with a variety of materials such as lanthanum hexaboride, strontium hexaboride, strontium oxide, barium oxide, zirconium carbide, or others having the desired work function, The walls are electrically heated as described in connection with housing 20. When the coated walls of housing cavity 25 come in contact with a material having a high electron aifinity, such as iodine, the neutral iodine atoms form negative ions.

Housing 23 is set at a potential of 2 kv. by applying the voltage to the housing walls through contact 29. Ground electrode 51, in this case, also serves as the extracting grid so that the extraction voltage applied to housing 23 is 2 kv. In contrast the extraction voltage applied to housing 18 is 4 kv. Consequently the geometry of the housing cavity 25 must be somewhat different (larger for example) from the geometry of housing cavity 20 in order for the density of the negative ions leaving the housing cavity 25 to be the same as the density of the ions leaving housing cavity 20. It is noted that the voltage difference between the potential of housing cavity 23 and the second electrode 51 is 2 kv. so that the negative ions will have the same specific impulse as the positive ions.

The individual tubes and housings may be combined in an array 54 and distributed in uniformly spaced relation to each other (see FIG. 5). The tubes 12 in housings 18 are divided into two groups. In addition, there are two material supply sources such as ovens 26 and 27 (see FIG. 6). As described above, the oven or material supply source 26 contains cesium while the oven or material supply source 27 contains a material such as iodine. Ends 14 of the group of tubes connected to the housings having tungsten walls would terminate inside oven 26 while ends 14 of the group of tubes connected to the housings having cavity walls coated with lanthanum hexaboride for example, would terminate inside oven 27. The reaction between cesium and the tungsten walls of the housing cavity 20 results in the formation of positive cesium ions while the reaction between the coated walls of housing cavity 25 and the iodine atoms produces negative ions, as described above. Consequently, the housings connected with one group of tubes will form positive ion beams and the housings connected with the other group of tubes will form negative ion beams. In order to extract the negative ions from the housing cavities, the polarity or sign of the electrical field applied between electrode 51 and exit aperture 24 would have to be opposite to that required for the extraction of the positive ions from the housing cavities.

In order for the ion propulsion motor to function, the charge of the ion beam ejected from the motor must be neutralized. This can be done by inserting an electron stream in the ion beam (in the case of positive ions). However, the mass of the electrons is so small that they contribute little to the thrust. On the other hand, by neutralizing the positive ion beam by a heavier negative ion beam as described in this embodiment, the thrust of the propulsion device can be substantially increased. It is apparent, therefore, that the number of housing cavities producing positive ion beams and the number of housing cavities producing negative ion beams must be related to each other so that the combined ion beams ejected from the propulsion motor would be electrically neutral. For best results, it is desirable for the housings associated with both groups of tubes to be mixed together in a uniform manner so that the combined ion beam formed from a generally uniform mixture of narrow positive and negative ion beams, is more completely neutralized.

Obviously many modifications of the present invention are possible in the light of the above teachings. It is therefore to be understood that the invent-ion may be practiced other than as described and still remain in the scope of the appended claims.

We claim:

1. An apparatus for producing a high density ion beam comprising a housing, said housing having a cavity formed therein, an inlet and an exit portion communicating with the cavity in said housing, said inlet portion being adapted to receive a vapor means for raising the temperature of the walls of the cavity to such a level that contact between the vapor and the heated walls serves to ionize the vapor, said exit portion being small enough to retain substantially all of said vapor atoms in said housing and an electrode outside the housing generally facing said exit portion, means for applying a potential to said electrode to provide an electric field between the electrode and the housing and thereby causing a portion of said electric field to penetrate through said exit portion into said housing cavity.

2. Apparatus for producing a high density ion beam as in claim}, in which the temperature of the cavity walls is raised to approximately l450 K.

3. An apparatus for producing a high density ion beam comprising a housing, said housing having a cavity formed therein, an inlet and an exit portion communicating with the cavity in said housing, said inlet portion adapted to receive a vapor having a predetermined ionization potential, means for raising the temperature of the walls of the cavity, the walls of the housing cavity formed from a material having a work function characteristic relative to the ionization ability of the vapor such that contact between the heated walls of the cavity and the neutral atoms of said vapor cause the vapor atoms to become ionized, the exit portion of said cavity being small enough so that substantially all the neutral atoms are retained in said cavity in posit-ion to be ionized whereby the ionizing efficiency of the apparatus is increased, and means for applying an electric field outside the housing and in such a position that a portion of said electric field penetrates the exit port-ion into the cavity of said housing, said electric field having a polarity such that said portion of said electric field penetrating said exit portion attracts the ions formed in the cavity and causes them to move out of the cavity in a focused narrow high density beam and the portion of said electric field outside the cavity further accelerates the ions in said beam.

4. The apparatus for producing a high density ion beam described in claim 3 wherein said means for applying an electric field outside the housing so a portion of said electric field penetrates the exit portion into the cavity of the housing, comprises an extracting electrode at a suitable potential placed in front of the exit portion of the housmg cavity.

5. An ion propulsion device comprising a housing at a predetermined electric potential and formed from a material having .a high work function, said housing having a cavity formed therein, an inlet portion and an exit portion communicating with said cavity, said inlet portion adapted to receive a vapor having a low ionization potential, means for raising the temperature of the walls of the cavity to a level high enough so contact between the neutral atoms of said vapor and the walls of said cavity cause the atoms to become ionized, creating thereby a positive space'charge in the cavity, the exit portion of said cavity being small enough so that substantially all of the neutral atoms of said vapor are retained in said cavity in position to be ionized whereby the ionization efiiciency of the propulsion device is increased, and an extracting electrode at a suitable potential placed in front of the exit portion to provide an electric field between the electrode and the exit portion, a portion of said electric field penetrating said exit portion into the cavity in said housing, said electric field having a polarity such that said portion of said electric field attracts the ions formed in the cavity and causes them to move out of the cavity through the exit portion, means for adjusting the electric field caused by the space charge in the cavity so that the space charge field cooperates with the portion of the electric field penetrating said cavity from said extracting electrode to reinforce the movement of the ions inside the cavity toward the exit portion whereby said ions move out of the exit portions of the cavity in a focused narrow high density ion beam where they are further accelerated by the electric field from said extracting electrode, a second electrode at local ground potential positioned beyond the extracting electrode whereby the potential ditference between said second electrode and the said housing cavity determines the specific impulse of the ions leaving the propulsion device, means for inserting charged neutralizing particles having a polarity opposite to said ions, into said ion beam for neutralizing said ion beam, said second electrode having the added function of preventing a counter-flow of said neutralizing particles into said housing cavity.

6. An ion propulsion device described in claim 5 including baffie means at the inlet portion of said cavity to distribute the incoming neutral vapor over the ionizing surfaces of the heated cavity walls.

7. An ion propulsion device comprising a housing at a predetermined potential below ground electric potential and formed from a material having a low work function, said housing having a cavity formed therein, an inlet portion and an exit portion communicating with said cavity in said housing, said inlet portion adapted to receive a vapor having a high electron afiinity relative to the work function of the housing material, means for raising the temperature of the walls of the cavity to a level high enough so contact between the neutral atoms of said vapor and the heated walls of said cavity cause the atoms to become ionized, creating thereby a negative space charge in the cavity, the exit portion of said cavity being small enough so that substantially all of the neutral atoms of said vapor are retained in said cavity in position to be ionized whereby the ionization efiiciency of the propulsion device is increased, and an extracting electrode at ground potential placed in front of the exit portion to provide an electric field between the electrode and the exit portion, a portion of said electric field penetrating said exit portion into the cavity in said housing, said electric field having a polarity such that said portion of said electric field attracts the ions formed in the cavity and causes them to move out of the cavity through the exit portion, means for adjusting the electric field caused by the space charge in the cavity so that the space charge field cooperates with the portion of the electric field penetrating said cavity from said extracting electrode to reinforce the movement of the ions inside the cavity toward the exit portion whereby said ions move out of the exit portion of the cavity in a focused narrow high density ion beam where they are further accelerated by the electric field from said extracting electrode, and means for inserting charged particles having a polarity opposite to said ions into said ion beams for neutralizing the ion beam.

8. An ion propulsion device comprising an elongated tube, a housing maintained at a predetermined electric potential formed from a material having a high work function, said housing mounted on one end of said tube and having a cavity formed therein with an inlet portion and an exit portion, said inlet portion communicating with said one end of said tube, an oven, the opposite end of said tube connected to said oven, oven heating means connected to said oven for raising its temperature whereby material in said oven having a low ionization potential is vaporized and flows through said tube into said housing cavity at a rate dependent on oven temperature, means independent of said oven heating means for raising the temperature of the walls of the cavity to a level high enough so contact between the neutral atoms of said vaporized material and the walls of said cavity cause the atoms to become ionized, creating thereby a space charge in the cavity, the exit portion of said cavity being small enough so that subsantially all the neutral atoms of the vaporized material are retained in said cavity in position to be ionized whereby the ionization efficiency of the propulsion device is increased, an extracting electrode at a suitable potential placed in front of the exit portion t provide an electric field between the electrode and the exit portion, a portion of said electric field penetrating said exit portion into said cavity, said electric field having a direction such that said portion of said electric field attracts the ions formed in the cavity and causes them to move out of the cavity through the exit portion, means in said cavity for adjusting the electric field caused by the space charge in the cavity so that the space charge field cooperates with the portion of the electric field penetrat ing said cavity from said extracting electrode to reinforce the movement of the ions inside the cavity toward the exit portion whereby said ions move out of the exit portion in the cavity in a focused narrow high ion density beam where they are further accelerated by the electric field from said extracting electrode, means for inserting charged particles having a polarity opposite to said ions into said ion beam for neutralizing it.

9. The ion propulsion device described in claim 8 including means for varying the temperature of the walls of the housing cavity and varying the oven temperature and hence the feed rate of the vapor flowing to said housing cavity to produce a variation in the number of ions produced and consequently provide means for varying the thrust of the propulsion device without varying the potential on the extracting grid.

10. The ion propulsion device described in claim 8 including a battle at the inlet portion of said heated cavity to distribute the incoming neutral vapor over the ionizing surfaces of the cavity walls.

11. An ion propulsion device as in claim 8, in which there is further provided a second electrode maintained at ground potential positioned beyond the first electrode whereby the potential difference between the second electrode and that of the housing determines the specific impulse of the ions leaving the propulsion device and prevents a counter fiow of the neutralizing particles into the housing cavity.

12. An ion propulsion device comprising an array of parallel elongated tubes distributed in uniformly spaced relation to each other, a housing mounted on one end of each tube, said housings divided into two groups, the housings in one group at a potential higher than ground potential and formed from a material having a high Work function, the housings in the other group at a potential lower than ground potential and formed from a material having a low work function, each of the housings having a cavity formed therein with an inlet portion and an outlet portion, each inlet portion communicating with one end of a tube, a first supply source of vaporous material having a low 0 ionization potential, a second supply source of vaporous material having a high electron affinity, the tubes connected to one group of housings connected to said first supply source of vaporous mate-rial, the tubes connected with the other group of housings connected to said second supply source of vaporous material, whereby vapor from the first supply source of material flows to one group of housings and vapor from the second supply source of material flows to the other group of housings, means for raising the temperature of the walls of the cavities in all the housings to a level high enough so that contact between the neutral vapor atoms from the first supply source and the walls of the cavities in the first group of housings form positive ions while the contact between the neutral vapor atoms from the second supply source and the walls of the cavities in the second group of housings forming negative ions, the exit portions of said cavities in all the housings being small enough so that substantially all the neutral atoms are retained in said cavities in position to become ionized whereby the ionization efliciency of the device is increased, an extracting electrode of suitable potential placed in front of each of the exit portions in all the housings to provide an electric field between each electrode and each exit portion, a portion of each electric field penetrating the exit portion into the corresponding housing cavity, each electric field having a polarity related to the presence of positive or negative ions in the associated cavity such that the said portion of said electric field attracts the particular ions in the cavity and causes them to move out of the cavity through the exit portion, means in each housing cavity for adjusting the electric field due to the space charge in the cavity so that the space charge field cooperates with the portion of the electric field penetrating said cavity from the associated extracting electrode to reinforce the movement of the ions in said cavity toward the exit portion, a second electrode at ground potential positioned beyond the extracting electrode in front of the housing cavities producing positive ions, the extracting electrodes in front of the housing cavities producing negative ions also at ground potential, whereby the ions in all the cavities move out of their respective exit apertures as focused narrow high-density beams, the potential connected to the first group of housings, the second group of housings and the potential on the electrodes selected to give the positive and negative ions the same specific impulse as they move out of the housing cavities, one ion beam group composed of positive ions and the other ion beam group composed of negative ions, the number of housing cavities producing positive ion beams being related to the number of housing cavities producing negative ion beams so that the combined ion beams are electrically neutral.

13. The ion propulsion device described in claim 12 wherein the group of tubes with their associated housings are arranged so they provide a generally uniform mixture of high density positive and negative ion beams.

References Cited by the Examiner UNITED STATES PATENTS 2,189,629 2/1940 Evans 313-339 2,215,787 9/1940 Hailer 313231 2,969,480 1/1961 Klein 313-63 3,015,745 l/l962 Klein 3l3231.5

DAVID J. GALVIN, Primary Examiner.

RALPH G. NILSON, GEORGE N. WESTBY,

Examiners.

C. R. CAMPBELL, Assistant Examiner. 

5. AN ION PROPULSION DEVICE COMPRISING A HOUSING AT A PREDETERMINED ELECTRIC POTENTIAL AND FORMED FROM A MATERIAL HAVING A HIGH WORK FUNCTION, SAID HOUSING HAVING A CAVITY FORMED THEREIN, AN INLET PORTION AND AN EXIT PORTION COMMUNICATING WITH SAID CAVITY, SAID INLET PORTION ADAPTED TO RECEIVE A VAPOR HAVING A LOW IONIZATION POTENTIAL, MEANS FOR RAISING THE TEMPERATURE OF THE WALLS OF THE CAVITY TO A LEVEL HIGH ENOUGH SO CONTACT BETWEEN THE NEUTRAL ATOMS OF SAID VAPOR AND THE WALLS OF SAID CAVITY CAUSE THE ATOMS TO BECOME IONIZED, CREATING THEREBY A POSITIVE SPACE CHARGE IN THE CAVITY, THE EXIT PORTION OF SAID CAVITY BEING SMALL ENOUGH SO THAT SUBSTANTIALLY ALL OF THE NEUTRAL ATOMS OF SAID VAPOR ARE RETAINED IN SAID CAVITY IN POSITION TO BE IONIZED WHEREBY THE IONIZATION EFFICIENCY OF THE PROPULSION DEVICE IS INCREASED, AND AN EXTRACTING ELECTRODE AT A SUITABLE POTENTIAL PLACED IN FRONT OF THE EXIT PORTION TO PROVIDE AN ELECTRIC FIELD BETWEEN THE ELECTRODE AND THE EXIT PORTION, A PORTION OF SAID ELECTRIC FIELD PENETRATING SAID EXIT PORTION INTO THE CAVITY IN SAID HOUSING, SAID ELECTRIC FIELD HAVING A POLARITY SUCH THAT SAID PORTION OF SAID ELECTRIC FIELD ATTRACTS THE IONS FORMED IN THE CAVITY AND CAUSES THEM TO MOVE OUT OF THE CAVITY THROUGH THE EXIT PORTION, MEANS FOR ADJUSTING THE ELECTRIC FIELD CAUSED BY THE SPACE CHARGE IN THE CAVITY SO THAT THE SPACE CHARGE FIELD COOPERATES WITH THE PORTION OF THE ELECTRIC FIELD PENETRATING SAID CAVITY FROM SAID EXTRACTING ELECTRODE TO REINFORCE THE MOVEMENT OF THE IONS INSIDE THE CAVITY TOWARD THE EXIT PORTION WHEREBY SAID IONS INSIDE THE CAVITY EXIT PORTIONS OF THE CAVITY IN A FOCUSED NARROW HIGH DENSITY ION BEAM WHERE THEY ARE FURTHER ACCELERATED BY THE ELECTRIC FIELD FROM SAID EXTRACTING ELECTRODE, A SECOND ELECTRODE AT LOCAL GROUND POTENTIAL POSITION BEYOND THE EXTRACTING ELECTRODE WHEREBY THE POTENTIAL DIFFERENCE BETWEEN SAID SECOND ELECTRODE AND THE SAID HOUSING CAVITY DETERMINES THE SPECIFIC IMPULSE OF THE IONS LEAVING THE PROPULSION DEVICE, MEANS FOR INSERTING CHARGED NEUTRALIZING PARTICLES HAVING A POLARITY OPPOSITE TO SAID IONS, INTO SAID ION BEAM FOR NEUTRALIZING SAID ION BEAMS, SAID SECOND ELECTRODE HAVING THE ADDED FUNCTION OF PREVENTING A COUNTER-FLOW OF SAID NEUTRALIZING PARTICLES INTO SAID HOUSING CAVITY. 